In the information society of these days, electronic display devices are more important as information transmission media and various electronic display devices are widely applied for industrial apparatus or home appliances. Such electronic display devices are being continuously improved to have new appropriate functions for various demands of the information society.
In general, electronic display devices display and transmit various pieces of information to users who utilize such information. That is, the electronic display devices convert electric information signals outputted from electronic apparatus into light information signals recognized by users through their eyes.
In the electronic display devices dividing into an emissive display device and a non-emissive display device, the emissive display device displays light information signals through a light emission phenomena thereof and the non-emissive display device displays the light information signals through a reflection, a scattering or an interference thereof. The emissive display device includes a cathode ray tube (CRT), a plasma display panel (PDP), a light emitting diode (LED) and an electroluminescent display (ELD). The emissive display device is called as an active display device. Also, the non-emissive display device, called as a passive display device, includes a liquid crystal display (LCD), an electrochemical display (ECD) and an electrophoretic image display (EPID).
The CRT has been used for a television receiver or a monitor of a computer as the display device for a long time since it has a high quality and a low manufacturing cost. The CRT, however, has some disadvantages such as a heavy weight, a large volume and high power consumption.
Recently, the demand for a new electronic display device is greatly increased such as a flat panel display device having excellent characteristics, for example, thin thickness, light weight, low driving voltage and low power consumption. Such flat panel display devices can be manufactured according to the rapidly improved semiconductor technology.
An electroluminescent (EL) element is attracting attention of interested person as one of the flat panel displays. The EL element is generally divided into an inorganic EL element and an organic EL element depending on used materials.
The inorganic EL element is a device in which a high electric field is applied to a light emitting part and electrons are accelerated in the applied high electric field to collide with a light emitting center, so that the light emitting center may be exited to emit a light beam.
The organic EL element is a device in which electrons and holes are injected into a light emitting part from cathode and anode, respectively, and the injected electrons and holes are combined with each other to generate excitons, thereby emitting light when these excitons are transited from an excited state to a base state.
Owing to the above operation mechanism, the inorganic EL element needs a high driving voltage of 100-200 V, whereas the organic EL element operates at a low voltage of 5-20 V. The above advantage of the organic EL element is activating researches on the organic ELD. Also, the organic EL element has superior properties such as wide viewing angle, high response speed, high contrast and the like.
The organic EL elements can be applied to both of the active matrix type display device and the passive matrix type display device. The active matrix organic EL display device is a display device that independently drives EL elements corresponding to a plurality of pixels using switching elements such as a thin film transistor. The organic EL display device is also referred to as an organic electroluminescent display (OELD) device or an organic light emitting device (OLED). Hereinafter, the active matrix organic EL display device is referred to as AMOLED device.
FIG. 1 is an equivalent circuit diagram of a conventional AMOLED device.
Referring to FIG. 1, a unit pixel circuit of a conventional AMOLED device includes two thin film transistors TFT1 and TFT2, and one capacitor Cst.
Specifically, a plurality of gate lines g1 and g2 and a plurality of data lines d1 and d2 are arranged to cross each other, thereby defining a unit pixel region. Between the adjacent data lines d1 and d2, there is arranged a direct current signal line Vdd to be parallel to the data lines d1 and d2. A maximum value of a display signal is applied to the Vdd line in the form of direct current.
A first thin film transistor TFT1 as a switching element is connected at a cross point of the gate line g1 and the data line d1. A gate electrode of the TFT1 is connected to the gate line g1 and a source electrode of the TFT1 is connected to the data line d1.
Between a drain electrode of the TFT1 and the Vdd line, there is connected a storage capacitor Cst. Also, between the drain electrode of the TFT1 and the Vdd line, there is connected a second thin film transistor TFT2 as a driving element which is in parallel with the storage capacitor Cst. A gate electrode of the TFT2 is connected to the drain electrode of the TFT1, a source electrode thereof is connected to the Vdd line and a drain electrode thereof is connected to an organic EL element.
If the TFT1 is turned on, the TFT2 is turned on depending on a display signal value of the data line d1, so that a direct current signal value of the Vdd line is applied to the organic EL element, thereby driving the organic EL element.
However, in the above AMOLED device to which the circuit using the two TFTs is applied, there occurs a problem in that brightness of the panel becomes non-uniform due to a deflection in the characteristics of the driving TFT, for example, variation in the threshold voltage.
In order to compensate for the deflection in the characteristics of the driving TFT, there were proposed compensation circuits to which a separate thin film transistor is added. However, in case that these compensation circuits are applied, the number of the thin film transistors increases and thus, an area occupied by the thin film transistors in a unit pixel region increases to thereby decrease the aperture ratio. Further, since a part of the compensation circuits needs two capacitors having different functions from each other, the number of the thin film transistors and the capacitors arranged in a unit pixel increases to cause the reduction of the aperture ratio. The reduction in the aperture ratio lowers the brightness, and requires a high current driving so that the life of the circuit is shortened.